1. Field of invention
This invention relates to the field of telephony, particularly forming a cross point switch using Microlectromechanical systems (MEMS) structures mounted on a ceramic substrate.
2. Description of the Related Art
One aspect of the art of telephony involves interconnecting any of a plurality of inputs, N, to any of a plurality of outputs M. It is desired to connect an input signal, part of N, carried on an incoming line, to an output, part of M, carried on an outgoing going line. This function is generally performed by an N×M matrix of crosspoints capable of interconnecting any member of N to any member of M. Such a structure of M×N crosspoints is a coordinate switch, that is, a rectangular array of crosspoints in which one side of the crosspoints is multiplied in rows and the other side in columns. A crosspoint is ideally a two state switching device having low transmission impedance in one state and very high in the other, as defined by the Bell System Technical Journal, September 1964. When the number of crosspoints provided within the switch is N×M, the coordinate switch has full availability and is non-blocking, that is, at all times there is a path between any pair of idle lines connected thereto, regardless of the number of paths already occupied.
A crosspoint in early mechanical telephone exchanges was a copper bar sliding between two contacts for providing the necessary interconnection between an input and an output as well as isolation between input and output. With the advent of electronic telephone exchanges, such as Bells' Electronic Switching System (ESS line), the switching function was performed by solid state devices such as Field Effect Transistors, Metal Oxide Field effect transistors, CMOS gates and the like. Unfortunately, as the size of the N×M matrix grows, it becomes more difficult to provide one single, massive, stage to interconnect all inputs and outputs. Instead, for example, a multi stage network is implemented to reduce the total number of crosspoints as compared to the N×M matrix. T. L. Bowers, in Derivation of Blocking Formulae for 3 Stage “Folded” switching arrays IEEE paper No CP 63-1462 calculates the number of crosspoints for a reduced (3 stage) configuration. The limitation to the number of stages is that, unlike the single stage design, the switched signal has to traverse a plurality of crosspoints within the exchange. Thus, if the basic element of a switch, the crosspoint, is lossy, traversing a series concatenation of them may attenuate the signal to a level where it is no longer usable, or the signal to noise ratio reduced below acceptable levels.
One approach for reducing the attenuation of a switching element is to use MEMS type structures having low insertion loss to perform the switching function. Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common substrate through the utilization of microfabrication technology. While typical electronics elements such as transistors are fabricated using integrated circuit (IC) process sequences (e.g., CMOS, Bipolar, or BICMOS processes), micromechanical MEMS structures are fabricated using process compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
An example of a MEMS structure having a bidirectional rotating member having two positions is described in U.S. Pat. No. 6,072,686, incorporated herein by reference in its entirety. Another example of a MEMS structure for microwave (millimeter wave) applications is described in U.S. Pat. No. 6,046,659, incorporated herein by reference in its entirety.
With the advent of higher operating frequencies, forming a switching matrix for use in telephony using MEMS structures requires special considerations subject of this invention.